1. Field of the Invention
The present invention relates to an electric field light-emitting device or an electro-luminescence (EL) device adapted for use as a flat panel display device or a light source therefor, or as a one or two-dimensional light-emitting device.
2. Related Background Art
The electroluminescence device has been developed as a flat light source, and is attracting attention in recent years because of improvements in the luminance and service life characteristics, relative ease of preparation of a large-area device with satisfactory uniformity of luminance, a small thickness, a light weight and possibility of flexibility in device.
Also such electroluminescence device is attracting attention in the office automation-related applications such as a rear illumination for liquid crystal display, a charge eliminating light source for a plain paper copying machine, and other applications requiring a high contrast, a high resolving power and a wide viewing angle.
In such conventional thin film EL device, the matrix crystal of the light emitting layer is composed for example of ZnS, ZnSe, CaS or SeS. Also the light emitting centers are composed for example of rare earth elements, transition metals, halogen atoms, alkali metal, alkali earth metals, or compounds containing these elements according to the colors to be generated.
However, the light emitting film of such thin film EL device, which is conventionally formed by resistance heating, electron beam evaporation or sputtering, is particularly susceptible to humidity for example due to structural defects of the film and is associated with the drawbacks of unstable light emission and film deterioration, so that a moisture insulating film, covering also the end faces of the device, has been indispensable.
For solving such problem of moisture resistance, an EL device in which the light emitting layer is composed of a diamond-state carbon film was disclosed in Appl. Phys. Lett. [53(19) Sept. 1988].
Such EL device is superior in the moisture resistance to the EL devices with conventional materials, but is still associated with drawbacks such as a high applied voltage, a low luminance and a short service life.
Also among the conventional EL devices utilizing ZnS or ZnSe, those utilizing ZnS:Mn for yellow-orange color emission are known to be of a high luminance and a long service life, but those utilizing Tm, Cu-Cl or Cu-Al as the light emitting centers for blue color emission are associated with the drawbacks of low luminance and short service life.
Electroluminescence of diamond was also reported for example by J. R. Briole and F. C. Champion [Proc. Phys. Soc. p.849-859, 80 (1962)]on granular diamonds of about 1 mm in size.
The use of diamond in the light emitting layer at first provides an advantage of a band gap as large as 5.4 eV, which allows a wide selection of elements and compounds usable as the light emitting center, and enables blue light emission.
Also the diamond has an insulation voltage equal to or higher than 1 MV/cm so that the electrons for exciting the light emitting centers can be sufficiently accelerated, and a high luminance can be attained by the efficient excitation of the light emitting centers. Besides the diamond is chemically stable, and not easily affected by humidity or atmosphere, so that a device of long service life can be obtained.
However granular diamond crystals of about 1 mm in size are unable to form a high density matrix in the light emitting layer.
Also such diamond is not suitable for mass production of EL devices of uniform luminance. In addition, since the diamond requires an extremely high voltage for light emission, it is not suitable for practical use as the device for display requiring a large area.
Furthermore, the diamond, when used as the matrix crystal of the light emitting layer, can increase the number of atoms or compounds constituting the light emitting centers, thus enabling multi-color emission due to the band gap as large as 5.4 eV.
However the light emitting layer composed of granular diamond crystals of about 1 mm in size is associated with the drawbacks of difficulty of doping of elements constituting the light emitting centers, difficulty of forming a high density matrix and inability for forming a flat panel or a two-dimensional light emitting device.
Also, a thin diamond film formed by conventional gaseous synthesis is a polycrystalline film having surface irregularities of several thousand Angstroms. Also a small amount of amorphous carbon or graphite may be present for example in the crystal grain boundaries, depending on the film forming method.
If such thin diamond film is employed as the light emitting layer of an EL device, with electrodes directly attached thereto, there may be locally applied a high electric field depending on the surface topography, eventually leading to destruction of insulation, or the insulation voltage the diamond layer is lowered due to the impurities present therein or the element or compound doped as the light emitting centers, eventually leading to the destruction of insulation under a high electric field, so that the performance of the device is inevitably unstable.
Such diamond film with uneven surface inevitably results in irregular and unstable contact between said thin film and the electrode, whereby a local high electric field is generated at the voltage application.